This invention relates to a modulator for use in a mobile telecommunication system.
In general, a mobile telecommunication system comprises a plurality of radio telephone terminals and at least one base station. Each of the radio telephone terminals has a quadrature modulator for quadrature modulating an input digital signal into a transmitting signal which is transmitted to the base station. The transmitting signal may be transmitted to the base station through one of channels. The input digital signal may be, for example, a digital speech signal.
Such a quadrature modulator is disclosed in a title of "Digital Implementation of Orthogonal Modulator for FM" contributed by H. Suzuki et al to IEICE Technical Report, No. CS79-250, 1979, pages 31 to 36. The quadrature modulator disclosed by H. Suzuki et al comprises a complex vector modulator section, a radio frequency (RF) section, and a combining section. The complex vector modulator section will be merely called a modulator hereinunder. The modulator is supplied with the input digital signal and produces an inphase component signal and a quadrature component signal on the basis of the input digital signal. The inphase and the quadrature component signals may be collectively called an output signal. The RF section frequency converts the inphase component signal and the quadrature component signal into a converted inphase component signal and a converted quadrature component signal, respectively. The combining section combines the converted inphase component signal and the converted quadrature component signal to produce the transmitting signal.
The input digital signal is composed of a plurality of symbols. In order to restrict a band width which the transmitting signal occupies and in order to minimize intersymbol interference and inter-channel interference, the modulator carries out Nyquist waveform shaping process of an impulse response of each symbol on producing the inphase component signal and the quadrature component signal as well known in the art.
By the way, it is known in the art that time division multiple access (TDMA) is used in the mobile telecommunication system. In TDMA, one of time slots is assigned as a specific time slot to a specific one of the radio telephone terminals. When the specific radio telephone terminal transmits the transmitting signal to the base station in the specific time slot, the specific radio telephone terminal transmits the transmitting signal as a burst signal to the base station.
On transmitting the burst signal, the inter-channel interference inevitably occurs at a start of the burst signal and an end of the burst signal because a spectrum becomes wide in the burst signal. In addition, inter-slot interference inevitably occurs in the base station because the spectrum becomes wide in the burst signal by a receiving filter of the base station.
In order to decrease the inter-channel interference and the inter-slot interference, it is known in the art to carry out a wave shaping of the burst signal at the start and the end of the burst signal by a predetermined window function. The predetermined window function may be, for example, Hanning window function.
A conventional modulator comprises a read-only-memory (ROM) for memorizing first through N-th impulse response data each of which is defined by an impulse response of phase information of the input digital signal, where N represents a positive integer which is not less than one. Namely, each of the first through the N-th impulse response data is defined by an impulse response function. More particularly, the first impulse response data is produced by multiplying the impulse response and a first coefficient which is weighted by the use of the predetermined window function. Similarly, the N-th shaping data is produced by multiplying the impulse response and an N-th coefficient which is weighted by the use of the predetermined window function.
In order to carry out the wave shaping of the burst signal at the start and the end of the burst signal, the first through the N-th impulse response data are selectively read as a read impulse response data out of the ROM in accordance with the input digital signal. Namely, the conventional modulator produces the output signal by the use of the read impulse response data.
However, it is necessary for the conventional modulator to make the ROM have a large capacity inasmuch as the ROM memorizes the first through the N-th impulse response data. Namely, the ROM itself becomes large.
As a result, the conventional modulator becomes large inasmuch as the ROM itself becomes large. Furthermore, the ROM is frequently accessed inasmuch as the ROM memorizes first through the N-th impulse response data. Accordingly, consumption of an electric power increases in the conventional modulator.